The present invention relates to a magnetic recording medium suited for high-density recording.
In the magnetic recording media such as VTR tapes, audio tapes and computer tapes, the necessity is rising recently for higher recording density in order to satisfy the requests for higher performance, longer recording time and smaller lightweight. Shortening of recording wavelength is essential for realizing high-density recording, but when the magnetic layer has a greater thickness than a certain level in relation to the recording wavelength, the flux of the magnetic substance in the deep layer is unable to pass the head and consequently forms a closed loop. Thus, this magnetic flux turns up a loss, since it does not pass the head core. In principle, therefore, the shorter the recording wavelength is, the smaller should be the thickness of the magnetic layer.
In line with this concept, there has been developed a so-called ME (metal evaporation)-type magnetic recording medium in which the recording layer is strikingly reduced in thickness by vapor depositing or sputtering a ferromagnetic metal in order to realize a remarkable reduction of the thickness loss resulting from shortening of recording wavelength. This ME-type magnetic recording medium, however, is at a disadvantage in that the thin ferromagnetic metal film composed of cobalt, nickel or a mixture thereof as magnetic substance tends to rust. Although various improvements have been made, such magnetic recording medium still lacks reliability for the long-time retention of recordings.
In the case of using MP (metal powder)-type magnetic recording medium in which a magnetic substance of a ferromagnetic metal dispersed in a resin binder is coated on a base film, it is relatively easy to cope with rusting, for instance, the rusting can be prevented by forming an anti-oxidant film on the metal particle surfaces. However, in the case of using the coating-type magnetic recording media, certain problems in production such as formation of pinholes or streaks, tend to arise when the thickness of the magnetic layer is reduced. It is also a hindrance to thickness reduction of the magnetic layer that the packing ratio of the magnetic substance can not be increased because of using a resin binder containing various additives.
For overcoming these disadvantages of the coating-type magnetic recording media, there has been proposed a magnetic recording medium in which a magnetic layer composed of two layers is coated on the base film, the lower non-magnetic layer being formed on the base film and then the upper magnetic layer being formed on the on said non-magnetic layer (Japanese Patent Application Laid-Open (KOKAI) No. 4-271016 and 5-28464). However, when the magnetic layer is coated immediately after the lower non-magnetic layer is coated on the based film, there tends to take place disarrangement of interface between the lower non-magnetic layer and the upper magnetic layer, resulting in generation of tape modulation noise. Even when using a method in which the lower non-magnetic layer is first coated and then, while this layer is still in a wet state, the upper magnetic layer is coated thereon, it is still impossible to perfectly prevent such disarrangement of interface therebetween. Thus, there has been room for improvement in the coating-type magnetic recording media.
As a result of intensive researches for solving these prior art problems, it has been found that by forming a magnetic layer comprising a magnetic substance dispersed in a resin binder on the surface of a polyester layer (polyester A layer ) constituting a laminated polyester base film which has been coextruded, so that the polyester A layer subject to plastic deformation by calendering treatment forms one of the surfaces thereof, the said magnetic layer having a thickness of 0.01 to 2.0 .mu.m, the said polyester A layer having a thickness of not less than 0.05 .mu.m, the sum of the thickness of the said magnetic layer and the thickness of the said polyester A layer being not more than 5 .mu.m, the obtained recording medium has an excellent flatness of the magnetic layer surface even when its thickness is reduced, and is very suited as a high-density magnetic recording medium for short wavelength recording. The present invention has been attained on the basis of this finding.